A variety of surveillance systems have been proposed for detecting vehicles, particularly aircraft. The most commonly utilized systems employ radar, which has the disadvantages of normally not providing accurate altitude information, being subject to defeat by counter measure techniques, being subject to attacks by radar homing devices, and providing no locatability of radio sources such as lightning bolts that do not originate from solid objects.
Several lower cost surveillance systems have been proposed which require high quality transmitters in aircraft for transmitting code modulated carrier waves. The carrier waves are detected at spaced ground stations, and the phase difference of the signal at an instant when it is detected by two stations indicates the difference in distance from the vehicle to each of the stations. This difference in distance defines a hyperbola. By utilizing two pairs of stations, two hyperbolic curves are defined, whose intersection is the location of the vehicle. These techniques have the disadvantage that they require at least moderately priced equipment in the aircraft, are not usable for detecting non-cooperating vehicles without such transmitters, and have not been useful in determining the altitude of vehicles.
There is a need for air space surveillance systems which do not have the limitations of existing radar systems, and which could complement existing radar systems or be compatible in operation with them. Ideally, such a surveillance system should be passive and three-dimensional. Additionally, such a system should not be subject to defeat by usual radar countermeasures.